Photographic processes using prescreened material



May 17, 1955 J. A. c. YULE ETAL PHOTOGRAPHIC PROCESSES USING PRESCREENEDMATERIAL Filed June 4, 1952 PICTURE EXPOSURE LINE EXPOSURE FLASHEXPOSURE AG/TATlO N STILL DEVELOPMENT ll j John/LC. Yule RicIzaniEMaarerINVENTORS Amnum United States Patent 0 PHOTOGRAPHIC PROCESSES USINGPRESCREENED MATERIAL John A. C. Yule and Richard E. Maurer, Rochester,

N. Y., assignors to Eastman Kodak Company, Rochester, N. Y., acorporation of New Jersey Application June 4, 1952, Serial No. 291,621

3 Claims. (Cl. 955) This is a continuation-in-part of application,Serial No. 168,718, filed June 17, 1950, now abandoned, having to dowith prescreened photosensitive material made by Clayden densitizationor by the combination of densitization. Reference is also made to twoother continuationsin-part thereof, Serial Nos. 291,622, and 291,623,filed concurrently with the present case, now issued as U. S. Patents2,691,585 and 2,691,586.

The present invention relates to processes using such materials for themaking of halftone images and for the making of line images without thehalftone effect.

Prescreened material produces a halftone image upon exposure to acontinuous tone picture. One object of the present invention is toreproduce line copy, i. e., lettering or line drawings on prescreenedmaterial.

A second object of the invention is to reproduce both line and picturecopy with the picture copy reproduced in halftone.

A third object of the invention is to extend the effective reproduciblerange or effective scale of the prescreened material in order toaccommodate long scale subjects. The effective scale of the prescreenedmaterial in terms of the density of the original subject is thedifference in density between the point of the subject which produces asuitable dot to record the highlights and one which produces a suitabledot to record the shadows in the halftone material. This effective scaleis a property of the prescreened material. It may be extended by flashexposure or still development as discussed below. The primary purpose ofextending the scale is to accommodate a subject whose density rangeexceeds that of the unextended effective scale of the material alone.However, it is also useful in holding down the contrast of thereproduction process in those special cases wherein the original subjecthas a very short density range and the operator wishes to reproduce thisshort density range exactly. Many printing processes have an overallgamma greater than one so that a pencil sketch, for example, is normallyreproduced more contrasty than is proper if the finished result is tolook like a pencil sketch. It is in these special cases that the flashexposure and/or still development are used to extend the scale of theprescreened material beyond that of the subject itself. In general,according to the invention, the effective scale is extended until it isat least substantially as great as the density scale of the picturebeing copied.

To accomplish all of the above objects, the present invention employs anumber of discoveries concerning the effect of high and low intensityexposures, the effect of short and long duration exposures, the effectof successive exposures, the effect of controlled agitation duringdevelopment, and particularly the effect of these factors on the scaleor contrast range being reproduced. For clarity, these various factorsare now discussed and de fined separately.

Clayden desenitization as used in making the prescreened materialrequires a short duration exposure. For example, one may employ a vaporflash lamp high intensity exposure through a ruled screen to provideClayden desensitization in the corners of the dots of the halftonepattern. The duration of such flash lamp exposures is exceedingly short.There is a cutoff point or region above which the exposure is no longereffective in giving Clayden desensitization. This cutoff point differswith different materials and may be on the order of or in some unusualcases, as long as one second exposure. The intensity selected is thatwhich gives substantially maximum densitization at the dot corners. Asone proceeds from the corner of the dot toward the center, thedesensitization is progressively less, and at areas adjacent to thecenter there is substantially no desensitization. In some forms ofClayden prescreened material the centers of the dots are provided withhypersensitization by a relatively low intensity long exposure, but thepresent invention is not particularly concerned with the presence orabsence of hypersensitization in the dot centers. There are some areasadjacent to the dot centers, whether this includes the exact center ornot, which have substantially no desensitization and which can befurther desensitized.

The hypersensitizing exposure may have been merely of sufficientintensity to provide maximum hypersensitization without any actuallatent image being formed, i. e., the exposure may be just less thanthreshold, or the centers of the dots may include an actual tiny latentimage.

The next type of exposure to be discussed is herein referred to as ablitz exposure. This exposure actually serves to eliminate theprescreening and to restore the film to its original, non-prescreenedcondition although the effective speed of the film may be slightlydifferent and there may be a residual pattern, for example, ofhypersensitization, which is more or less harmless. The blitz exposureis generally of the same order of duration and intensity as the Claydendesensitizing exposure, but it is not through a halftone screen. Inthose areas of the prescreened material which receive the blitzexposure, the corners of the dots, which have previously receivedmaximum desensitization, become more sensitive again due to the addedexposure. On the other hand, the areas adjacent to the centers of thedots which have received little or no Clayden desensitization throughthe original screen, are now Clayden desensitized slightly. It turns outin practice that the film becomes substantially uniform in sensitivitythroughout the whole blitzed area. That is, the sensitivity is flattenedout by a high intensity short exposure, which explains in two ways whythe term blitz has been adopted for this particular exposure.

Optimum Clayden densensitization does not make the sensitive layercompletely insensitive. It reduces the sensitivity thereof by a factorwhich, in log E units, corresponds to the effective scale of thematerial as discussed below. Additional Clayden exposure by theabove-mentioned blitz carries the effect beyond its optimum and henceincreases the reduced sensitivity of the corners very slightly. At thecenters and in the intermediate areas, the blitz exposure reduces thesensitivity.

The ordinary image exposure when using the regular prescreened material,is normally of long duration and must not be too short or there will besome flattening of the sensitivity pattern which will result in inferiorreproduction. That is, the image exposure must exceed the cutoff point(e. g., V second) of the material being used. The exposure must be ofsufficient intensity and sufficiently long duration to expose thelightest areas of the copy fully without desensitizing any areas of thesheet. This brings up a rather curious effect which, in a sense, isalternative to a main feature of the invention to be discussed below inconnection with the reproduction of line copy. In accordance with thisalternative effect, line work may be reproduced directly on prescreenedmaterial merely by making the exposure to the line work a high intensityshort duration exposure, but in this case the intensity has to beextremely high, e. g., by contact printing. Hence, this alternativemethod is not as satisfactory as the blitz method discussed below.

Still another type of exposure used in certain embodiments of thepresent invention is the flash exposure used for extending the range ofthe prescreened film in order to reproduce long scale subjects. Thisflash exposure has the same order of intensity and duration as the imageexposure. It must not be a short duration high intensity exposure suchas the blitz exposure. The effect of the flash exposure is such thatthis exposure may be either before or after the picture exposure onordinary prescreened film. The blitz exposure on the other hand is notused with pictures but only with line work and must always be prior tothe exposure of the line work.

The scale or density range of the material is further extended incertainembodiments of the invention by still development, i. e., bydeveloping with no agitation or with agitation only for a short intervalfollowed by still development for the remainder of the developmentperiod. The flash exposure and still development factors for extendingthe effective scale of the prescreened material are similar in manyrespects to the same steps when used in ordinary halftone processesemploying ordinary film and ruled screens or contact screens. Theseordinary processes sometimes also employ an image exposure without thescreen to shorten the scale in the highlights, but in the presentprocess the screen cannot be removed except when blitz effects are usedand so this type of scale shortening is not directly and easilyapplicable to the present invention. One important distinguishingfeature between screen processes and prescreened material processes, asfar as extension of scale is concerned, is the fact that withprescreened materials the duration or time of exposure becomes some whatcritical since extremely short exposures affect the prescreeningadversely as far as image and scale extension are concerned. Also, thescale which is to be extended depends on the distribution ofsensitivities rather than on a distribution of brightnesses as is thecase with contact and ruled screens. It turns out that the relationshipsfollow similar mathematical laws, however.

According to the invention, one starts with Clayden prescreenedphotographic sheet material such as film or plate with the dot cornersClayden desensitized and with areas adjacent to the dot centers havingsubstantially no desensitization. This material is used for reproducingline work by first exposing the sheet or selected area thereof uniformlyto a high intensity short duration exposure of sufiicient intensity andsufiiciently short duration to Clayden desensitize the areas adjacent tothe dot centers and on the other hand to reduce the desensitization ofthe dot corners. This provides substantially the same sensitivity in theareas adjacent to the centers and in the corners. The sheet is thenexposed to the line work which is to be reproduced and processed to forma line negative of the line Work.

In a preferred embodiment of the invention, both line and picture copiesare reproduced on- Clayden prescreened material of the above-discussedtype by masking off the areas which are to receive the pictureandtgiving the blitz exposure to the remainder areas. The remainderareas are then exposed to the line copy and the picture areas areexposed to the picture copy either separately or at the same time. Theexposure of the picture copy is the more critical one and should be ofsufficiently long duration to avoid Clayden desensitization. In generalit should exceed second. The film or sheet is then processed to form aline and halftone picture negative of the line and picture copy.Preferably, the picture areas of the sheet are also given a uniformflash exposure either before or after exposure to the picture image, theflash exposure being of sufiicient intensity and sufiiciently longduration to extend the effective scale of the sheet. This flash exposuremay also be given to the line copy areas, if given subsequent to theblitz exposure. Preferably, also, the sheet is developed at least partlyby still development to extend the effective scale still further.Specifically, if the effective scale of the sheet unextended is A, theextension of the scale due to the flash exposure is B, and the extensionof the scale to the still development is C, A+B+C is made at leastsubstantially as great as the density scale of the picture copy. Ingeneral it is made substantially equal to this density scale in order toreproduce the copy fully. However, in those special cases mentionedpreviously, it is sometimes made to exceed this scale so as to reproducea low scale subject accurately with a high gamma printing system.

Although the use of flash exposures and still development withprescreened material differs from its use with contact screens, theefiect on scale obeys the same mathe: matical laws which are describedin detail in:

Tone reproduction in halftone negatives by J. A. C. Yule, Proceedings ofAnnual Technical Meeting, Technical Association of the LithographicIndustry, 1950, page 67.

Improved halftones with Kodak magenta contact screens. Eastman KodakCompany, October 1950.

Instructions for making halftone negatives. Eastman Kodak Company,February 1951.

When exposing prescreened material by contact printing, it has beenfound preferable to use a thin transparent spacer so that the grain ofthe image does not interfere with the structure of the individual dots.

The invention will be fully understood from the following description ofa preferred embodiment of the invention when read in connection with theaccompanying drawing which shows:

A flow chart schematically illustrating all of the preferred features ofthe invention.

In the drawing a prescreened film consisting of a support 10 and asilver halide emulsion layer 11 which has been Clayden desensitizedthrough a halftone screen to have an undulating sensitivity halftonepattern, is masked by an opaque mask 13 so that only area 12 of theprescreened film is exposed. The exposure of area 12 is a blitz exposureby a high intensity vapor flash lamp 14. This blitz exposure effectivelyremoves the prescreening from the area 12 although there may be a slightresidual dot pattern, especially if the prescreening was partly byhypersensitization, but even so, this dot pattern is practicallyharmless in the area 12 which is to be used for reproduction of linework.

In the second step of the drawing the prescreened and partly blitzedfilm is exposed through a lens 21 to copy 16 whose area 17 includes onlylettering and whose area 13 includes a picture which is to be reproducedby halftone. The copy is illuminated by are lamps 19. The lettering inarea 17 is focused on the blitzed area 12 of the prescreened film 11.The picture area is exposed onto the regular prescreened area 22 of thefilm.

In the third step of the process, the effective scale of the prescreenedmaterial in the halftone area 22 is extended by a flash exposure. Theremaining areas including the blitzed area 12 are masked off by a mask23. As

stated above, it is not absolutely necessary to mask off the blitzedarea but it is preferable to do so. The flash exposure is provided by alow intensity lamp 24-, about 7 /2 watts through a yellow Series 00Safelight filter 25 at a distance of 6 to 8 feet, the exposure being onthe order of 12 seconds with prescreened materials of the lith type suchas discussed in our two applications mentioned above. The film is thenplaced in a lith or surface type developer 26 (such as the typedescribed in U. S. Patent 2,313,523, Donovan et al.) and is processedfirst with agitation for about one minute and then with stilldevelopment for the remainder of the development period which totalsabout 2 /4 minutes with standard lith developers. The period ofagitation is in general standardized and may, for example, be 45seconds, 1 minute, 1%: minutes, or 1 /2 minutes. The standardized stilldevelopment provides a standard amount of scale extension and the flashexposure is then selected to give whatever additional scale extension isdesired. Flash exposures have been used in halftone work for many yearsand the methods of using them are well known. For ex tremely criticalwork one may follow the method of determining the precise flash exposureas set forth in Tone Reproduction in Halftone Negatives by J. A. C.Yule, mentioned above.

In general it is preferable not to extend the overall development timewhen employing still development since excessive development has atendency to destroy the halftone effect all or" which ties in with thefact that full realization of Clayden desensitization depends on thefact that surface developers are used, preferably developers of thesurface type which give infectious development.

On the other hand, slightly prolonged development may be used to shortenexposure range and increase highlight contrast in cases Where theaccompanying adverse efl ects are acceptable. As mentioned previously,this flash exposure and still development may be used either formatching the scale of the material to that of the subject or forreducing the scale of a low scale subject so that it is reproducedaccurately by a high gamma printing process.

The final step of the flow chart shows the dots 30 of a halttone imageand sections 31 of a line or letter image.

We claim:

1. The method of reproducing line work with a Clayden prescreenedphotographic sheet with dot corners Clayden desensitized and areasadjacent to the dot centers with substantially no desensitization, whichcomprises exposing the sheet uniformly to a high intensity shortduration exposure of substantially the same order of duration andintensity as that used in prescreening the film whereby said adjacentareas are Ciayden desensitized and the desensitization of said cornersis reduced to provide substantially the same sensitivity in said areasand said corners, then exposing the sheet to said line work anddeveloping the sheet.

2. The method of reproducing both line and picture copy with a Claydenpr screened photographic sheet with dot corners Clayden desensitized andareas adjacent to the dot enters with substantially no desensitization,which comprises covering wi h opaque material the areas of the sheetwhich are to receive picture, exposing the remainder areas of the sheetuniformly to a high intensity short duration exposure of substantiallythe same order of duration and intensity as that used in prescreeningthe film whereby said adjacent areas are Clayden desensitized and thedesensitization of said corners is reduced to provide substantially thesame sensitivity in said areas and said corners, then exposing saidremainder areas to said line copy and the picture areas to said picturecopy and developing the sheet, the duration of the exposure to saidpicture copy exceeding second.

3. The method according to claim 2 in which the picture areas are alsogiven a uniform flash exposure of sufiicient intensity and sufiicientlylong duration to extend the eifective scale of the sheet and in whichsaid developing is at least partly still developing to extend furtherthe ettective scale of the sheet.

References Cited in the file of this patent Eastman Kodak C0,, ImprovedHalftones with Kodak Magenta Contact Screens, 1950, 6 pp.

1. THE METHOD OF REPRODUCING LINE WORK WITH A CLAYDEN PRESCREENEDPHOTOGRAPHIC SHEET WITH DOT CORNERS CLAYDEN DENSENSITIZED AND AREASADJACENT TO THE DOT CENTERS WITH SUBSTANTIALLY NO DESENSITIZATION, WHICHCOMPRISES EXPOSING THE SHEET UNIFORMLY TO A HIGH INTENSITY SHORTDURATION EXPOSURE OF SUBSTANTIALLY THE SAME ORDER OF DURATION ANDINTENSITY AS THAT USED IN PRESCREENING THE FILM WHEREBY SAID ADJACENTAREAS ARE CLAYDEN DESENSITIZED AND THE DESENSITIZATION OF SAID CORNERSIS REDUCED TO PROVIDE SUBSTANTIALLY THE SAME SENSITIVITY IN SAID AREASAND SAID CORNERS, THEN EXPOSING THE SHEET TO SAID LINE WORK ANDDEVELOPING THE SHEET.